Drapery actuator

ABSTRACT

A drapery actuator, including a motor drive for a drapery pull cord, detects a closed or open limit condition for the draperies through detecting a predetermined tension condition in the pull cord. In response to predetermined tension, the motor drive is reversed for a short period for relieving the tension in the pull cord.

United States Patent Ruben A. Gill 615 S.W. Country Club Road, Lake Oswego, Oreg. 97034 July 14, 1969 Feb. 9, 1971 Inventor Appl. No. Filed Patented DRAPERY ACTUATOR 14 Claims, 6 Drawing Figs.

US. Cl 160/331, 254/173; 318/267 Int. Cl r. A47h 5/02 Field of Search... 160/331; 226/183; 254/173, 14]; 318/267, 266, 466, 469

[5 6] References Cited UNITED STATES PATENTS 3,269,454 8/1966 Gill et al.

Primary Examiner-David J. Williamowsky Assistant ExaminerPhilip O. Kannan AttorneyBuckh0m, Blore, Klarquist and Sparkman ABSTRACT: A drapery actuator, including a motor drive for a drapery pull cord, detects a closed or open limit condition for the draperies through detecting a predetermined tension condition in the pull cord. In response to predetermined tension, the motor drive is reversed for a short period for relieving the tension in the pull cord.

PATENTED FEB 9197: 3.661.520

JSHEET 1 or 2' RDBERT A. GILL INVENTOR BY BUC/(HOR/V, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS DRAPERY ACTUATOR BACKGROUND OF THE INVENTION In Gill et al. US. Pat. No. 3,269,454, a drapery actuator is set forth and claimed for opening and closing draperies or the like. The drapery pull cord is motor driven, and when a predetermined tension condition is detected in the pull cord, the motor drive is stopped. The motor drive ordinarily includes a gear reduction unit which tends to maintain a pull cord tension after the drive motor is inactivated. Due to the inertia of the motor mechanism and gear reduction unit, the motor is also apt to overrun whereby the retained tension is increased from the detected value. Wear and tear on the pull cord and the other mechanism sometimes results, especially in the case of heavier drapery installations.

SUMMARY OF THE INVENTION According to the present invention, motive means are employed for opening and closing closure members such as draperies or the like, and predetermined limit conditions are detected when the closure members are either entirely opened or entirely closed. Such limit conditions are advantageously detected from the degree of tension in a closure member pull cord. In response to the detection of a limit condition, the motive means is temporarily reversed for relieving the force or tension which may be exerted upon the closure-member and/or pull cord. Since reversal is fairly rapid in response to a predetermined limit condition, application of excessive force or tension is substantially lessened, and the pull cord is placed in slack or tensionless condition. I 7

It is an object of the prevent invention to provide an improved reversible automatic actuator for closure members such as draperies or the like.

It is another object of the present invention to provide an improved drapery actuator adapted for automatic operation of closure members wherein wear and tear on such closure members and their accessories are materially reduced.

It is a further object of the present invention to provide an improved actuator for heavy duty closure members.

It is another object of the present invention to provide an improved actuator for closure members wherein such actuator is adapted to relieve stress and strain upon the closure member operating mechanism and accessories by relief of the force or tension exerted thereon.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. 1 illustrates an actuator in accordance with the present invention as installed for operating a set of draperies;

FIG. 2 is a vertical section of the actuator;

FIG. 3 is a horizontal section looking upward along the line 3-3 in FIG. 2, the view being on an enlarged scale;

FIG. 4 is a schematic diagram of a first actuator circuit according to the present invention for bringing about the relief of force or tensionin the closure actuator;

FIG. 5 is a schematic diagram of a second circuit of the present invention employing solid state devices; and

FIG. 6 is a schematic diagram of a further circuit according to the present invention.

DETAILED DESCRIPTION Referring to the drawings, a set of draperies l0 and II are illustrated as mounted in the usual manner on a drapery rod 12 fixed to a wall I3. In the conventional arrangement of such devices a pull cord is affixed to the draperies l0 and I l in such manner that the pull cord is formed into a bight 14 with taut reaches 15 and 16 which extend vertically in substantially parallel rclation, one reach being attached to thc draperies in such manner that pulling thereon opens the draperies and the other reach being attached to the draperies in such manner that pulling thereon closes the draperies. An actuator comprises a housing 17 which may be attached to the wall 13 by screws 18, the housing being preferably provided with a hinged cover 19. The reaches 15 and 16 of the cord pass through a slot in the upper end of the cover. A power supply cable 21 enters the lower portion of the housing and is provided with a plug 22 for attachmcnt to an outlet receptacle 23. A branch cord 24 leads to a single pole, double throw control switch 25 which may be conveniently mounted on the wall at the normal height of wall switches.

The bight of the cord 14 embraces a drive roller 30 mounted upon a drive shaft 31 extending through a mounting plate 32 mounted within the housing 17. The drive shaft 3| extends through the mounting plate 32 into a reduction worm gear train (not shown) mounted on the rear of plate 32 and driven by a condenser run, reversible, induction type electric motor 35 fixed to the rear of the plate 32 by screws 36. The reaches of the cord are pinched between the drive roller 30 and a pair of idlenpressurc rollers 37 and 38 respectively. Each roller is provided with a resilient surfacing, roller 30 having a resilient surfacing indicated at 39, roller 37 having a resilient surfacing indicated at 40, and roller 38 having a resilient surfacing indicated at 4]. The pressure rollers are mounted on axes parallel to the axis of the drive roller and in such relation to the drive roller that the resilient covers are pressed together, the rollers being substantially on radii of the drive roller at apart and 45 to each side of an upper vertical radius centrally between the reaches 15 gripped 16. The thickness of the resilient coverings of the three rollers is such that the cord is firmly gripped therebetwecn and the relationship is such that the bight I4 is caused to engage a major portion of the periphery of the drive roller, in the illustrated instance somewhat in excess of 270 of the periphery. The cord is thereby firmly frictionally driven as the drive roller 30 is rotated in either direction by the action of the reversible motor.

Mounted above the pressure rollers are a double pole, double throw switch 50 and a switch actuator 51. The switch 50 preferably comprises a toggle device of conventional character including a pair of spring toggles 52 having overcenter springs therein for holding the toggles in either position, depending upon the shifting of a toggle plate 53 between limiting abutments 54. The toggle plate 53 is slotted for reception of the upper end of a spring lever 55 conveniently mounted in an insulator block 56 which holds the contacts of the switch. The spring lever 55 projects into a slot 60 in a switch actuator bar 61 which is slidably guided for limited horizontal movement in a pair of ears 62 standing above the plate 32. The ends of the actuator bar 61, which are guided in openings in the ears 62, are reduced so as to provide shoulders against which a pair of spring retainer washers 64 are seated. A pair of coil springs 65 surrounding the reduced portions are compressed between the cars 62 and the washers 64, and resist shifting movement of the switch actuator from a centralized position. The reaches 15 and I6 of the cord pass on the outer sides of a pair of eccentric guide bushings 66 which are mounted on the surface of the actuator by screws as illustrated, and which may be clamped in adjusted positions by the screws. The members 66 spread the reaches apart so that each reach leaves the grip of the rollers at an outwardly inclined angle, the angularity of each of which may be independently adjusted by appropriately setting the position of the members 66.

The device as thus far described corresponds to the device of the aforementioned patent. Its action is as follows. When one of the reaches 15 or 16 is pulled downwardly, the drapery or other closure member will reach a limit position, which will cause the motor to try to straighten the vertical angle therein caused by the guide members 66 on the switch actuator. Initially this will bend the flexible lever 55 until the force thereof against the toggle plate 53 overcomes the toggle springs 52, which thereupon causes the switch contacts to be thrown to the opposite position. When the position of switch 50 is thus reversed, the motor 35 will stop until the position of switch 25 is reversed, whereupon the motor will run in the opposite direction. As a result, the switch 25 can be set and the person may walk away knowing that as soon as resistance is encountered by reason of the drapery having reached the limit of its movement, the motor will be stopped.

Unfortunately, because of its weight, the rotor motor 35 has considerable inertia and when the motor is stopped, the gear train driven from the motor continues running for a short time in the same direction. For example, assume member 66 and switch actuator 51 are set such that switch 50 shuts off the power to motor 35 when a tension of 30 pounds is reached on the drapery cord. By the time the motor comes completely'to rest, the tension may well be on the order of 40 to 50 pounds. This tension tends to be held by the worm gear train employed in driving the drapery cord. Such tension can be destructive to the drapery cord, the resilient surfacing thereon, and to the motor gear train as well.

According to the present invention, when a predetermined tension is reached on the drapery cord, power is applied to rotate the motor in the reverse direction for relieving the tension from the cord and returning the cord to a slack condition. The reversing is accomplished quite rapidly whereby overtravel of the motor and gear train is lessened and the drapery cord is backed off to a completely slack position.

Referring to FIG. 4, illustrating circuitry according to the present invention for providing the above-described tension release, reversing motor 35 is provided with a clockwise terminal 70, a counterclockwise terminal 72, and a neutral terminal 74. The motor preferably is a series-wound universal (brush type) motor, but may be a capacitor type, and causes rotation of the motor in a clockwise direction for current flow between terminals 70 and 74, or alternatively in a counterclockwise direction for current flow between terminals 72 and 74. The terminals 70 and 72 are connected to outer fixed contacts 76 and 78 of double pole, double throw switch 50, adapted to be actuated by tension in the drapery pull cord as hereinbefore described. Movable contact 80, which makes connection with fixed contact 76 in the position shown, is connected to fixed contact 82 of a control switch 25, and the movable contact 84 of the latter switch connects to one side of an AC power line. The other fixed contact 86 of switch 25 is joined to movable contact 88 of switch 50, which makes connection with fixed contact 90 in the position shown. The motor neutral connection 74 connects to the other side of the power line.

A relay 92 includes an operating coil 94 and normally open contacts 96 and 98. In energized condition, operating coil 94 holds contacts 96 and 98 closed as shown. Contact 90 of switch 50 is coupled via contacts 96 and diode 100 in series to terminal 70 of the motor, with the cathode of the diode being connected to terminal 70. Likewise, the remaining contact 102 of switch 50 is coupled via relay contacts 98 and diode l04in series to motor terminal 72, the cathode of diode 104 being connected to terminal 72.

One terminal of relay operating coil 94 is coupled through resistor 106 to the cathode of diode 108, while the anode of diode 108 is connected to one side of the line and motor ter minal 74. The remaining terminal of coil 94 is joined to the midpoint between two back-to-back connected diodes 110 and 112 wherein the cathodes of these diodes are respectively connected to motor terminals 70 and 72. Capacitor 114 is connected between one terminal of operating coil 94 and the end of resistor 106 opposite the other terminal.

Let us assume switches 25 and 50are in the position shown, e.g. for operating motor 35 in a clockwise direction and closing the draperies illustrated in FIG. 1. The motor will continue to run until switch 50 is operated by tension in the drapery cord. in the meantime, capacitor 114 will charge to a DC potential through diodes 108 and 110 whereby the right-hand side of capacitor 114 will be charged to a positive voltage relative to the left-hand side. Thecharging voltage normally maintains operating coil 94 in energized condition, and hence keeps contacts 98 closed so long as motor 35 continues to run. When the predetermined tension is reached in the drapery pull cord, switch 50 is operated as hereinbcforc described, stopping motor 35. Movable contact makes connection with' fixed contact 102 and movable contact 88 makes conncc tion with fixed contact 78, while the contacts of relay 92 are held closed by discharge of capacitor 114 through resistor 106 and operating coil 94; Therefore, motor 35 will be energized in the counterclockwise or reverse direction by current flowing through switch 25,,contacts 80 and 102, relay contacts 98, and diode 104. The motor current is thus shifted to the contacts of the relay. counterclockwise rotation will continue as long as the contacts of relay 92 remain closed, and those contacts will remain closed until discharge current from capacitor 114 flowing in coil 94 is insufficient to keep the contacts closed. The capacitance of capacitor 114 and the resistance of resistor 106 are chosen to provide an appropriate time constant so that contacts 96 and 98 will remain closed until motor 35 has rotated a predetermined number of revolutions in the counterclockwise direction. The higher the resistance and the larger the capacitance, the slower the discharge rate and the longer the motor remains energized. The motor output shaft is suitably thereby positioned from one-half to three turns in the reverse direction-from the position at which the output shaft originally came to a complete stop. Therefore, the tension on the drapery pull cord will be relieved to a substantially slack condition. The reversing or backing off current is only half wave which lessens the wear on the gears of the motor drive. That is, shock is avoided. Since the current tending to make the motor turn in a counterclockwise direction is substantially immediately applied to the motor when predetermined tension is reached, the motor is also substantially prevented from overrunning very far, and thus even the momentary application of excessive tension to the drapery pull cord is substantially avoided. Less than one-eighth turn of output drive shaft 31 in overtravel has been experienced before reverse rotation and backing off to a completely slack position. Even the tension used for detection purposes is relieved as the device is backed off to a slack position. As a result, wear and tear on the drapery pull cord arrangement, and the motor drive apparatus is materially reduced.

Of course, with switch 50 thrown to the right-hand position in FIG. 4 and with switch 25 remaining in the left-hand position, the draperies will then remain substantially at the position selected by switch 25. Now, if switch 25 is thrown to its right-hand position such that movable contact 84 makes connection with contact 86, the counterclockwise terminal of motor 35 will be energized through contacts 88 and 78 for a period of time long enough to open the draperies, for example. During this time, capacitor 114 charges through diodes 108 and 112, and relay contacts 96 and 98 are closed by operating coil 94. When the draperies are opened all the way such that a predetermined tensionoccurs in the pull cord, switch 50"will be operated whereby its contacts again assume the position illustrated in FIG. 4. Motor 35 will now be energized momentarily and in a clockwise direction inasmuch as a current will be provided through switch contacts 88 and 90, relay contacts 96, and diode 100. Clockwise rotation will continue only as long as the discharge from capacitor 114 holds the relay coil 94 energized. When the relay contacts open, the drapery pull cord will be substantially tensionless.

Referring to FIG. 5, solid state circuitry is illustrated foryaccomplishing motor reversal without employing mechanically moving relays or the like. Again, switch 50 is operated by tension on the drapery pull cord for disconnecting motor 35 when the pull cord has reached a predetermined limit. A bridge rectifier 118, which may be composed of semiconductor diodes, has one input terminal thereof connected to motor terminal 70 and the remaining input terminal thereof connected to motor terminal 74. Thus, bridge rectifier 118 is energized when motor 35 is operated in a clockwise direction. A voltage divider comprising resistors 120 and 122 is disposed across the positive and negative output terminals of bridge rectifier 118 to provide a reduced positive voltage at the junction of the two resistors with respect to the negative output terminal of the bridge. A diode 124 couples such junction to capacitor 126, with the remaining capacitor connection being returned to the negative terminal of bridge 118. The cathode of diode 124 is connected to the capacitor so that the diode end of the capacitor charges up to a positive voltage when the bridge rectifier provides an output.

The diode end of capacitor 126 is coupled through a resistor 128 to the base of NPN transistor 130, the latter having its emitter connected to the gate of silicon-controlled rectifier 132 via diode 134. The cathode of diode 134 is connected to the SCR gate. The collector of transistor 1.30 is coupled to the midpoint of a voltage divider comprising resistors 136 and 138 disposed in that order between the cathode of a diode 140 and the negative output terminal of bridge rectifier 118. The anode of diode 140 is connected to the anode of silicon-controlled rectifier 132 and to contact 102 of switch 50. The cathode of silicon-controlled rectifier .132 is coupled to terminal 72 of motor 35 via diode 104, wherein the cathode of diode 104 is connected to terminal 72.

Assuming switches 25 and 50 are in the position shown, motor 35 will turn in a clockwise direction until such time as switch 50 is operated by tension in the drapery pull cord. During this time, bridge rectifier 118 charges up capacitor 126 via voltage divider 120-122 and diode 124. When switch 50 is thrown to the opposite position by cord tension, a voltage is applied from switch contact 102 to silicon-controlled rectifier 132, and through diode 140 and resistor 136 to the collector of transistor 130. A relatively positive voltage is appliedto the base of transistor 130 from capacitor 126 and hence transistor 130 conducts, triggering silicon-controlled rectifier 132. Since an AC voltage is applied at the anode of silicon-controlled rectifier 132, the latter will continue to conduct essentially for as long as the capacitor 126 provides sufficient current through resistor 128 to maintain the conduction of transistor 130. Motor 35 will then turn briefly in the counterclockwise direction, and the duration of this reverse rotation is determined by the time constant of the capacitor 126-resistor 128 combination, as in the case of the capacitor l14-resistor 106 combination of the F IG. 4 embodiment.

1n the FIG. 5 circuit there is additionally provided a bridge rectifier 118', suitably composed of semiconductor diodes, having its input terminals connected between motor terminals 72 and 74. Thus, bridge rectifier 118 will provide an output as long as motor 35 is energized for counterclockwise rotation. Bridge rectifier 118' powers a circuit substantially similar to that supplied by bridge rectifier 118. Components having primed reference numerals at the right-hand side of FIG. 5 are substantially the same in construction and function as those bearing similar reference numerals on the left-hand side. However, the anode of silicon-controlled rectifier 132 connects to fixed contact 90 of switch 50 while the cathode of silicon-controlled rectifier 132' is coupled to motor terminal 70 via diode 100. Silicon-controlled rectifier 132' functions to cause momentary clockwise rotation of the motor 35 after a counterclockwise rotation of the motor and operation of switch 50, thus completing a circuit for providing momentary reverse rotation and tension release after a motor run in either direction.

The circuit of FIG. 6 is essentially similar to the FIG. 5 circuit except that higher rated transistors 142 and 144 are employed in place of the silicon-controlled rectifiers. The FIG. 5 circuit is preferred over the FIG. 6 circuit at the present time because of the limitations in voltage and current ratings of power switching transistors readily available at a reasonable cost figure. However, the FIG. 6 circuit is operable either employing more costly transistors, or is operated at reduced voltage.

In the FIG. 6 circuit, elements similar to those in FlG. 5 are identified by corresponding reference numerals. The base of transistor 142 is connected to the positive side of capacitor 126 via resistor 128 while the collector of transistor 142 is connected to switch contact 102 via diode 146 having its anode connected to contact 102. The emitter ottransistor 142 is coupled by way of, diode 104 to motor terminal 72. Similarly, the positive side of capacitor 126' is coupled to the base of transistor 144 via resistor 128', and diode 148 connects the collector of transistor 144 to switch contact 90. The anode of diode 148 is connected to contact 90. The emitter of transistor 144 is coupled to motor terminal 70 employing diode 100.

The circuit is similar in operation to that of FIG. 5. Assuming switches 25 and 50 are in the position shown, motor 35 will rotate in a clockwise direction. Capacitor 126 will charge up providing a positive voltage at the base of transistor 142.

Then, when a predetermined tension is detected in the drapery pull cord, contact closes against contact 102 of switch 50, and transistor 142 is energized via diode 146. Transistor 142 provides current to motor terminal 72 through diode 104 so long as capacitor 126 continues to supply sufficient current to transistor 142 for maintaining the conduction of the transistor. When capacitor 126 substantially discharges through resistor 128, transistor 142 and the motor windings, transistor 142 will no longer deliver current to motor terminal 72. The time constant of the circuit principally comprising capacitor 126 and resistor 128 is chosen such that motor operation will take place in a counterclockwise direction for a short period of time, such time being sufficient for relieving the pull cord tension.

When the motor is initially operated in a counterclockwise direction, the operation of the circuit including transistor 144 is substantially similar to the above described sequence.

When the term closure has been employed in the present application, a set of draperies have been referred to as a principal example. However, the present invention is not limited in its use to the control of draperies, but may also be employed for operating other closure members such as doors, windows, movable room dividers, adjustable lou-vers. shutters, folding partitions, and the like.

While 1 have shown and described preferred embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects.

1 claim:

1. In an actuator for opening and closing closure members:

motive means for moving a said closure member between predetermined limits of travel;

means for detecting a predetermined limit of travel condition of said closure member; and

circuit means responsive to said detection for reversing said motive means for a short period of time compared with the time of closure operating movement between limits of travel, for relieving the force applied to said closure member by said motive means.

2. The actuator according to claim 1 wherein a limit of travel condition is detected by detection of force exerted relative to said closure member.

3. The actuator according to claim 1 including a pull cord through which said motive means drives said closure member, and wherein said means for detecting said limit of travel condition comprises means for detecting a predetermined tension in said pull cord.

4. In an actuator for opening and closing closure members:

a pull cord for moving such closure members;

a reversible electric means for driving said cord;

first switch means movable in response to tension in said cord between a first condition for actuating the reversible electric means in one direction and a second condition for actuating said reversible electric means in a second direction;

control switch means for energizing said first switch means for actuating said electric means only in a selected one of said directions for bringing about unilateral movement of said closure member; and

further means for additionally energizing said first switch means for a limited period of time immediately following movement of said first switch means, for causing said electric means to reverse direction temporarily and relieve tension on said pull cord.

5. The actuator according to claim 4 wherein said further means comprises an electric time constant circuit for bringing about energization of said rcversibleelectric means substantially for a time proportional to the duration of discharge of such time constant circuit, and contacts on said first switch means through which said electric means is actuated in said reverse direction for said limitedperiod of time.

6. The actuator according to claim 5 wherein said further means additionally includes a relay operated by said time constant circuit and through which said reversible electric means is energized for said limited period of time.

7. The actuator according to claim 5 wherein said further means additionally includes a semiconductor device activated by said time constant means and through which said reversible electric means is energized for said limited period of time.

8. The actuator according to claim 5 including circuit means for charging said time constant circuit during energization of said reversible electric means by said first switch means in a selected direction.

9. The actuator according to claim 4' wherein said control switch means is provided with first and second contacts for selecting first and second directions for said reversible electric means and wherein said further means is disposed in shunt relation with nonselected contacts of said control switch means for energizing said reversible electric means in said reverse direction for said limited period of time through addi tional contacts provided on said first switch means.

10. ln an actuator for opening and closing closure members:

reversible motor means including a gear reduction drive for moving a said closure member; I

a control switch for said motor means, said control switch having forward and reverse contacts;

limit switch for detecting limit conditions of said closure member and having contacts operable in combination with contacts of said control switch'for reversing said motor from a given direction to a reverse direction when a limit condition is reached and when said control switch is also operated to direct reverse'movemcnt' of said closure member; and

a circuit responsive to actuation of said limit'switch for limited energization of said motor means in said reverse direction through further contacts of said limit switch before said control switch is also operated, said circuit including time constant means setting such limited energization of said motor means to a predetermined period of time.

11. "The actuator acc'ordin'g to claim 10 wherein said time constant means comprises a capacitor-resistor discharge circuit, and means for charging said capacitor during operation of said motor means in said given direction.

12. The actuator according to claim 10 wherein said circuit includes a relay operated by said time constant means for completing a connection through a said further contact of said limit switch for energizing said motor means.

13. The actuator according to claim 10 wherein said circuit includes a silicon-controlled rectifier having its gate coupled to said time constant means for causing said silicon-controlled rectifier to complete a connection through a said further contact of said limit switch for energizing said motor means.

14. The actuator according to claim 10 wherein said circuit includes a transistor having a control terminal thereof coupled to said time constant means for causing said transistor to complete a connection through a said further contact of said limit switch for energizing said motor means. 

1. In an actuator for opening and closing closure members: motive means for moving a said closure member between predetermined limits of travel; means for detecting a predetermined limit of travel condition of said closure member; and circuit means responsive to said detection for reversing said motive means for a short period of time compared with the time of closure operating movement between limits of travel, for relieving the force applied to said closure member by said motive means.
 2. The actuator according to claim 1 wherein a limit of travel condition is detected by detection of force exerted relative to said closure member.
 3. The actuator according to claim 1 including a pull cord through which said motive means drives said closure member, and wherein said means for detecting said limit of travel condition comprises means for detecting a predetermined tension in said pull cord.
 4. In an actuator for opening and closing closure members: a pull cord for moving such closure members; a reversible electric means for driving said cord; first switch means movable in response to tension in said cord between a first condition for actuating the reversible electric means in one direction and a second condition for actuating said reversible electric means in a second direction; control switch means for energizing said first switch means for actuating said electric means only in a selected one of said directions for bringing about unilateral movement of said closure member; and further means for additionally energizing said first switch means for a limited period of time immediately following movement of said first switch means, for causing said electric means to reverse direction temporarily and relieve tension on said pull cord.
 5. The actuator according to claim 4 wherein said further means comprises an electric time constant circuit for bringing about energization of said reversible electric means substantially for a time proportional to the duration of discharge of such time constant circuit, and contacts on said first switch means through which said electric means is actuated in said reverse direction for said limited period of time.
 6. The actuator according to claim 5 wherein said further means additionally includes a relay operated by said time constant circuit and through which said reversible electric means is energized for said limited period of time.
 7. The actuator according to claim 5 wherein said further means additionally includes a semiconductor device activated by said time constant means and through which said reversible electric means is energized for said limited period of time.
 8. The actuator according to claim 5 including circuit means for charging said time constant circuit during energization of said reversible electric means by said first switch means in a selected direction.
 9. The actuator according to claim 4 wherein said control switch means is provided with first and second contacts for selecting first and second directions for said reversible electric means and wherein said further means is disposed in shunt relation with nonselected contacts of said control switch means for energizing said reversible electric means in said reverse direction for said limited period of time through additional contacts provided on said first switch means.
 10. In an actuator for opening and closing closure members: reversible motor means including a gear reduction drive for moving a said closure member; a control switch for said motor means, said control switch having forward and reverse contacts; limit switch for detecting limit conditions of said closure member and having contacts operable in combination with contacts of said control switch for reversing said motor from a given direction to a reverse direction when a limit condition is reached and when said control switch is also operated to direct reverse movement of said closure member; and a circuit responsive to actuation of said limit switch for limited energization of said motor means in said reverse direction through further contacts of said limit switch before said control switch is also operated, said circuit including time constant means setting such limited energization of said motor means to a predetermined period of time.
 11. The actuator according to claim 10 wherein said time constant means comprises a capacitor-resistor discharge circuit, and means for charging said capacitor during operation of said motor means in said given direction.
 12. The actuator according to claim 10 wherein said circuit includes a relay operated by said time constant means for completing a connection through a said further contact of said limit switch for energizing said motor means.
 13. The actuator according to claim 10 wherein said circuit includes a silicon-controlled rectifier having its gate coupled to said time constant means for causing said silicon-controlled rectifier to complete a connection through a said further contact of said limit switch for energizing said motor means.
 14. The actuator according to claim 10 wherein said circuit includes a transistor having a control terminal thereof coupled to said time constant means for causing said transistor to complete a connection through a said further contact of said limit switch for energizing said motor means. 